As originally published Eliot's note for this line was:
It was later changed to:Shantih. Repeated as here, a formal ending to an Upanishad. 'The Peace which passeth understanding' is a feeble translation of the content of this word.
Shantih. Repeated as here, a formal ending to an Upanishad. 'The Peace which passeth understanding' is our equivalent to this word.